Monocrystalline calcium fluoride (CaF2, flourite) exhibits high transmittance of a wide range of wavelengths of light, ranging from the vacuum ultraviolet region to the infrared region. Therefore, monocrystalline CaF2 is widely used in various types of optical members, such as in the arc tubes of excimer lamps (Japanese Unexamined Patent Application Publication No. 2009-163965A), for example.